Structural components for the boiler zone of power plants or refuse incineration plants

ABSTRACT

The invention relates to structural components for the boiler zone of power plants or refuse incineration plants, especially tubes or nests of tubes, fin and diaphragm walls, that consist of steel or steel alloys, and is characterized by a continuous closed outer nickel or nickel alloy layer galvanically deposited having a layer thickness of approximately 1 to 5 mm, preferably 1 to 2.5 mm.

BACKGROUND OF THE INVENTION

The invention relates to components for the boiler area of powerstations or refuse incineration plants inclusive of special wasteincineration plants, as well as such plants in which corrosive, abrasiveloading or the like can lead to premature damage.

Since combustion equipment such as power stations or refuse incinerationplants fulfill the purpose always only in conjunction with upstream ordownstream equipment such as conveyors, heat exploitation and flue gaswashing installations, the operational readiness of all individualcomponents is decisive for overall availability. In the case ofcombustion of solid fuels or solid industrial or special wastes as wellas domestic refuse, plants of that kind usually consist of deliverysystems for the solid fuels, a furnace in the form of a drum furnace orshaft furnace, optionally with an afterburning chamber, a steam boilerfor waste heat utilization, a flue gas washer and the chimney. Thecombustion temperatures in the furnace lie between about 800 to 1200° C.for a dwell time of up to one hour for the combustion material. Thearising flue gases have to be cooled from the combustion chamber by wayof the boiler flues towards the dust extraction installations, whereinthe continuous temperature difference can be up to 800° C. Since fluegas as raw gas can contain up to more than 10,000 mg/m³ of dust in theraw gas and always still up to 100 mg/m³ of dust in the clean gas, adeposit of these flue gas constituents at the water-conducting orsteam-conducting heat carrier system, such as pipes, pipe bundles,finned walls, etc., in the average temperature range cannot be avoided.The dust components bake into stone-like coatings on the components ofsteel or steel alloys, which on the one hand obstructs heat transfer andon the other hand very strongly promotes corrosion, since HCl and othersubstances contained in the flue gas are also contained in the coatingsand there lead to chloride corrosion of the steel. Apart from thechemical attack by corrosion, there is also, however, a mechanicalattack by abrasion through the dust particles contained in the flue gas,wherein abrasive wear can also come about with, for example, the threatof rust by the combustion material itself.

Components such as pipes or pipe bundles therefore have to be regularlydemounted and replaced, due to abrasive or corrosive wear, in boilerareas in power stations or refuse incineration plants, which generallymeans temporary stoppage of the entire plant and which causes annualcosts for each plant in the order of magnitude of several hundreds ofthousands of Deutsch Marks, in conjunction with an uncoordinatedavailability. It has already been attempted to deal with this problem bythe use of other materials such as, for example, new forms of nickelalloys; thus, EP 0 819 775 describes a nickel-chromium-iron alloy with areduced content of niobium, which shall have good corrosion resistanceand workability. There is known from EP 0 928 658 an Ni—Cr—Mo—Fe alloywhich can be welded to a metal support. This alloy, too, shall have ahigher corrosion resistance than the usual alloys. A further Ni—Cr—Coalloy with increased corrosion resistance is described in PCTapplication WO 97/43457. For the repair of metal pipes with wearlocations, EP 0 729 522 describes application, by electroplating, to theinner wall of such a pipe of commercially available alloys based on, forexample, nickel, chromium, copper or iron. Wear locations in pipes witha typical diameter between 10 to 50 mm can be repaired in this manner,wherein the repair locations can be approximately 5 to 900 mm long. Inaddition, the repair alloy applied by electroplating must satisfyspecific conditions with respect to layer thickness and compound crystallimits. Moreover, there are known from PCT application WO 98/09751 pipesfor incineration plants which consist of an inner pipe component ofspecific steel alloys and an outer component of a special Ni—Cr—Mo—Nb—Fealloy, wherein the two components are combined by way of a metallurgicalbond, particularly by the Osprey method.

The special alloys proposed in the publications are comparativelyexpensive, so that they are hardly used in practice. Only theresurfacing weld method also described in, for example, EP 0 729 522,which is also termed “cladding”, has found some applications inpractice, but it has proved that the service lives can be improved inthe hoped-for degree only to a limited extent, and moreover a welding ofelbows and straight pipes is required.

There therefore still exists a need for components for the boiler areaof power stations or refuse incineration plants and the like which havea significantly higher resistance against corrosive or abrasive wear andcan be produced in a production process without additional weld seams.

SUMMARY OF THE INVENTION

For solution of the task there are proposed components for the boilerarea of power stations or refuse incineration plants, particularly pipesand pipe bundles, made of steel or steel alloys, which are characterizedby a continuous outer electroplated layer of nickel or nickel alloyswith a layer thickness of approximately 1 to 5 mm, preferably 1 to 2.5mm.

Quite unexpectedly it has now been established that a continuousexternal electroplated layer of pure nickel or nickel alloys,particularly Ni—Co alloys, with a relatively small layer thickness ofapproximately 1 to 2.5 mm suffices to multiply the service lives of suchcomponents. Such a nickel coating applied by electroplating obviouslyprevents not only abrasive wear in all temperature ranges, but quiteunexpectedly the deposition of airborne particles of dust on thecomponents and consequent chloride corrosion are thereby largelyprevented in the average temperature range. Tests in a refuseincineration plant have shown that the rate of erosion in the case ofpipes treated in accordance with the invention was virtually equivalentto zero by comparison with unprotected pipes, which in the same periodof use at the same location exhibit an erosion of up to 1.5 mm within aquarter of a year. Since hardly any dust particles had settled on thesurface of the pipes, a significantly reduced expenditure on cleaningcan also be expected in the future.

Nickel-plating belongs to the oldest method of electroplating technologyand has been used in different variants for a long time. The componentof steel or steel alloys to be electroplated is in that case connectedas a cathode; electrolyte nickel is not usually used as the anodematerial for nickel-plating, since due to passivation this woulddissolve only in certain electrolytes. Processing is normally with amixture of nickel sulfate and small quantities of nickel chloride aswell as boric acid or citric acid. The methods used are state of the artand known to one skilled in the art.

The outer coating applied by electroplating can comprise pure nickel oralso—according to the respective temperature requirement and otherenvironmental requirements—nickel alloys; where, for example, there is athreat of rust, the use of an Ni—Co alloy is preferred due to itsgreater wear resistance. Similarly, Ni—Mo, Ni—P or alloys with aspecific content of rare earth metals can also be used for specificpurposes.

The nickel-plating can be undertaken not only on pipes and pipe bundles,but also on other plant components such as serpentine heating pipes,finned walls, diaphragm walls, etc.

Nickel-plating is a method enabling a largely stress-free coating. It iseven possible, if required, to connect plant components by weldingwithout in that case damaging the nickel-plating.

The particular advantage of the invention resides above all in the factthat the service lives of the loaded components in the boiler area canbe multiplied and thereby the otherwise inevitable shutdowns aresimilarly significantly reduced and standstill periods can be planned. Adirect statement still cannot be made about the level of cost reductionsuch as in the case of a 320,000 ton plant like, for example, theRugenberger Dam with two boilers (each boiler 160,000 tons), sinceempirically established values for that are lacking.

However, it can be assumed that the increased cost due to nickel-platingwould already be amortized after two years. A service life improvementof at least 8 to 10 years is calculated.

Beyond that it has been shown in tests that the advantageous componentshave a further effect which is manifested in that the components remainfree especially from the particles which otherwise usually settle orthese particles substantially no longer adhere to the components. Intechnical terms the special effect achieved is also denoted by theso-called lotus flower effect.

Moreover it has proved that with the preferred form of special coatingthe efficiency of a power station or a refuse incineration plant can bedecisively enhanced by the improved thermal transfer in consequence ofcomponents which are kept clean. In addition, due to the improvedefficiency the overall heat output can be decreased and thus the exhaustof environmentally harmful CO₂ gas significantly reduced.

What is claimed is:
 1. Components for the boiler area of power stationsor refuse incineration plants, wherein said components are made of steelor steel alloys, and have a single continuous, closed, outerelectroplated layer of nickel or nickel alloys, and wherein said layerhas a thickness of approximately 1 to 5 mm.
 2. Components according toclaim 1, wherein said components are selected from the group consistingof pipes, pipe bundles, finned walls and diaphragm walls.
 3. Componentsaccording to claim 1, wherein said layer has a thickness of from 1 to2.5 mm.
 4. Components for gas conduction according to claim 1, whereinsaid outer layer comprises a Ni—Co alloys or Ni—Mo alloy.
 5. Componentsfor gas conduction accord to claim 1, wherein said outer layer comprisesNi—P alloys or Ni—Si-metal alloys.
 6. Components for gas conductionaccording to claim 1, wherein said layer has a thickness ofapproximately 1.5 mm.